The invention relates to multifocal spectacle lenses. Such lenses have a dioptric power varying according to the zone of vision on the lens, and are typically used for spectacle wearers suffering from presbyopia.
Multifocal lenses comprise lenses known as progressive lenses adapted to vision at all distances. These lenses usually comprise a torical or spherical surface, that may be adapted to the wearer of the spectacle lenses, and an aspherical surface chosen from a family of surfaces. Each point of an aspherical surface is usually characterised by a mean sphere S and by a cylinder C. Mean sphere S is defined from the formula ##EQU1##
in which:
R.sub.1 and R.sub.2 are the maximum and minimum radii of curvature expressed in meters, and PA1 n is the refractive index of the lens material. PA1 where R.sub.1 and R.sub.2 are maximum and minimum radii of curvature expressed in meters, and n is the refractive index of the lens material, PA1 wherein, for a given direction of sight, the absolute value of the difference between a binocularity parameter for two points in the object space is as small as possible, PA1 said binocularity parameter being defined, for a point (M) in the object space as the relative difference .DELTA.S of the mean sphere for the points (M.sub.D, M.sub.G) of the aspherical surface of the right and left lenses through which the wearer sees said point (M). PA1 where S.sub.D and S.sub.G are the values of mean sphere at said points (M.sub.D, M.sub.G) of the aspherical surface of the right and left lenses through which the wearer sees said point (M).
With the same definitions, cylinder C is given by the formula: ##EQU2##
Progressive multifocal ophthalmic lenses comprise a far vision region, a near vision region, an intermediate vision region, and a main meridian of progression passing through the three regions. For such lenses, the addition value A is defined as the variation in mean sphere between a reference point in the far vision region and a reference point in the near vision region.
Progressive multifocal ophthalmic lenses also comprise a main meridian of progression, also called principal line of sight; it is a line usually defined as the intersection of the line of sight with the aspherical surface of each lens when the wearer of the lenses fixes a point in the object space in front of him, at various distances.
French patent application FR-A-2 699 294 comprises in its preamble more detailed definitions of the various elements of a progressive multifocal ophthalmic lens (main meridian of progression, far vision region, near vision region, power addition value, etc..); it also describes the work carried out by the applicant to improve wearer comfort of such lenses.
One of the problems for multifocal lenses is the taking into account of binocularity. Indeed, human vision is the result of the combination of vision through two eyes, or fusion of the images provided by the two eyes. When the image of a point of the object space on the retina of the right and left eye is at two corresponding or homologous points, the images provided by both eyes are combined, so that the person wearing the spectacle lenses only sees one object point. There may be binocular vision with a single object point even if the two points are not perfectly homologous points, provided they are not too far from being homologous.
One of the constraints facing the manufacturer of multifocal lenses is to design lenses that will provide appropriate power correction for one eye--that is provide appropriate power for any direction of sight-, and also allow proper fusion of the images of the two eyes, that is allow binocular vision.
For lenses of the prior art that have symmetry with respect to the main meridian of progression, it is usual to partially rotate the lens by about 10.degree. when fitting the lenses in the spectacle frame, so as to accommodate the accommodation convergence of the eyes. This solution is a very rough estimate, and is not fully satisfactory for ensuring binocular vision.
U.S. Pat. No. 4,606,622 discusses the problem of fusion of the images provided by the two eyes of the wearer of multifocal spectacle lenses. This document notably discusses the problems of binocular vision in multifocal progressive lenses, and suggests to fit the lens with a non-straight principal line of sight. This line is inclined towards the nose at least in the near vision zone. The right and left lenses are symmetrical. For ensuring binocularity, it is suggested to consider lines of sight originating from the two eyes, for a given point in the object space, and to consider the curvature of the lens at the points of intersection of these lines with the two spectacle lenses; each line of sight extends on one of the temporal and nasal sides of a lens, and due to symmetry of the lenses, the difference in the curvature is thus only considered on one single lens. This document therefore suggests that the curvature of the lens be substantially symmetrical on opposite sides of the intercept of the principal line of sight to ensure a good foveal vision.
U.S. Pat. No. 5,666,184 also discusses the problem of binocularity, and suggests to limit, in the near vision portion, the difference in astigmatism on a horizontal line, between points that are symmetric with respect to the prime line of sight.
The solution of these two documents--asymmetrical design with a symmetry of astigmatism with respect to the principal line of sight--may be appropriate for static vision: the difference between the images of a point in the object space is sufficiently limited for allowing binocular vision in the far and near vision zone of a multifocal lens, so that the lenses ensure a good foveal vision in these zones.
However, this solution does not bring a solution to the problem of dynamic vision, or vision of the wearer of the spectacle outside of the near and far vision zone. A number of wearers cannot adapt to multifocal lenses due to problems in dynamic vision, that may originate in bad or inappropriate binocular vision.